Conventionally, as a flow rate measuring apparatus for measuring a mass flow rate of a gas, widely known is a differential pressure type of flowmeter having a pitot tube, an orifice plate or the like, an ultrasonic type of flowmeter having an ultrasonic sensor, a laminar flow type of flowmeter having laminar flow elements with a plurality of tubes, a hot wire type of flowmeter having a heater (hot wire), and the like.
However, in the case of measuring a mass flow rate of an exhaust gas from an engine of a car and the like, the flow rate measuring apparatus mentioned above should not be used.
This is because the exhaust gas has properties that (1) matters (e.g. water drops into which vapor condenses, unburned fuel, and engine oil) other than a gas component are present therein, (2) a flow velocity thereof widely varies depending on a temperature, a rotational speed of the engine, and the like, (3) pressure pulsation proportional to the rotational speed and the number of cylinders of the engine occurs, and (4) a flow thereof becomes imbalanced because of a bending portion of an exhaust duct, and these properties of the exhaust gas shown at (1) to (4) disable the flow rate measuring apparatus mentioned above from accurately measuring the mass flow rate of the exhaust gas.
Patent Literature 1 discloses an apparatus which performs PIV (Particle Image Velocimetry) processing to analyze fluid flowing through a duct.
The apparatus disclosed in Patent Literature 1, at each of two points in time apart from each other at a minute time interval, irradiates the inside of the duct through which the fluid flows with a sheet-like laser, and at the same time, takes an image of a section of the internal space of the duct cut by the sheet-like laser. Then, the apparatus calculates movement amounts of particles of the fluid on the basis of the images taken at the two points in time. By using the movement amounts of particles of the fluid calculated in this manner, a mass flow rate of the fluid flowing through the duct can be calculated.
Though the apparatus disclosed in Patent Literature 1 can take an image of any place in the duct, in the case of measuring the mass flow rate of an exhaust gas, the apparatus cannot accurately measure the mass flow rate of the exhaust gas because of no consideration of imbalance of fluid flow.